A biochemical mechanism by which activated macrophages injure neoplastic cells is being studied in cell culture. Macrophage-induced injury to murine leukemia L1210 target cells is manifest by arrest of cell division and inhibition of oxidative phosphorylation due to blocks at two sites in the mitochondrial electron transport chain. However, these target cells can escape death by generating chemical energy via glycolysis. Injured L1210 cells separated from macrophages were found to have acquired a new chemical reaction. In this reaction O2 is consumed by a non-mitochondrial mechanism, that is the O2 uptake is not inhibited by cyanide. The cyanide-insensitive O2 uptake reaction may be involved in cell injury because the intermediate products of O2 reduction (superoxide and hydrogen peroxide) are themselves toxic, or they may react to produce hydroxyl radical and singlet oxygen which are also toxic. L1210 cells, isolated after injury by macrophages, will be used to study this reaction. The specific aims are to determine: (a)\whether superoxide and/or hydrogen peroxide are produced within neoplastic cells after injury by macrophages and whether this correlates with the inhibition of mitochondrial electron transport which develops in these target cells. (b)\Whether inhibiting target cell defense mechanisms against O2 intermediates renders them more susceptible to macrophage injury. (c)\Whether peroxidation of membrane unsaturated fatty acids occurs during cyanide-insensitive O2 uptake. (d)\What the electron donor for this reaction is; whether there is a requirement for cofactors; and what catalyzes the reaction. While it is well known that activated macrophages can reduce O2 to superoxide by an NAD(P)H oxidase, the possibility that neoplastic cells can be induced to undergo this reaction following contact with macrophages has not been considered. Understanding this mechanism of cell injury may provide a useful model for pharmacological engineering. Moreover, this knowledge may provide a means to augment host defnese mechanisms against neoplastic disease, and to interfere with the mechanisms by which neoplastic cells subvert macrophage-induced injury.